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SubjectRe: [patch V3 13/37] mips/mm/highmem: Switch to generic kmap atomic
On 2021-01-09 01:33:52 [+0100], Thomas Bogendoerfer wrote:
> On Sat, Jan 09, 2021 at 12:58:05AM +0100, Thomas Bogendoerfer wrote:
> > On Fri, Jan 08, 2021 at 08:20:43PM +0000, Paul Cercueil wrote:
> > > Hi Thomas,
> > >
> > > 5.11 does not boot anymore on Ingenic SoCs, I bisected it to this commit.
> > >
> > > Any idea what could be happening?
> >
> > not yet, kernel crash log of a Malta QEMU is below.
>
> update:
>
> This dirty hack lets the Malta QEMU boot again:
>
> diff --git a/mm/highmem.c b/mm/highmem.c
> index c3a9ea7875ef..190cdda1149d 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -515,7 +515,7 @@ void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot)
> vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
> BUG_ON(!pte_none(*(kmap_pte - idx)));
> pteval = pfn_pte(pfn, prot);
> - set_pte_at(&init_mm, vaddr, kmap_pte - idx, pteval);
> + set_pte(kmap_pte - idx, pteval);
> arch_kmap_local_post_map(vaddr, pteval);
> current->kmap_ctrl.pteval[kmap_local_idx()] = pteval;
> preempt_enable();
>
> set_pte_at() tries to update cache and could do an kmap_atomic() there.
So the old implementation used set_pte() while the new one uses
set_pte_at().

> Not sure, if this is allowed at this point.
The problem is the recursion
kmap_atomic() -> __update_cache() -> kmap_atomic()

and kmap_local_idx_push() runs out if index space before stack space.

I'm not sure if the __update_cache() worked for highmem. It has been
added for that in commit
f4281bba81810 ("MIPS: Handle highmem pages in __update_cache")

but it assumes that the address returned by kmap_atomic() is the same or
related enough for flush_data_cache_page() to work.

> Thomas.
>

Sebastian

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